Many reports have been published on techniques for detecting a tracking error signal in an optical head. Push-pull technique is well known as one of representative techniques and it is used practically.
An optical head using push-pull technique is explained below. In the optical head, a light emitted by a light source is condensed by an object lens to form a light spot on a plane of an optical disk for recording information on which a continuous groove of information track is formed spirally. Two photosensitive areas are provided by dividing a photosensitive area of the photodetector with a division line. The photodetector is shown in FIGS. 1-3 at the left side. The light reflected from the optical disk enters to the photodetector. Two photo-detecting signals from the two photosensitive areas are subjected to differential amplification to generate a tracking error signal. Tracking control is performed by controlling the position of the object lens in response to the tracking error signal. When the light spot is subjected to focus control, light intensity distribution of the reflected light is affected by diffraction at the continuous groove due to a position shift of the light spot from the groove. If the prior art optical head is used for an optical disk, light intensity distribution of the reflected light is known to be ascribed to interference of diffracted light beams of 0th, +1st and -1st orders at the continuous groove. Two hatched areas in circular light beam in FIGS. 1-3 denote interference regions. The light intensities in the two regions become asymmetrical according to the position shift of the spot from the continuous groove, and the differential signal is used as the tracking error signal.
The above-mentioned prior art optical head has a simple structure to detect a tracking detection signal. However, it has a problem that an offset of the tracking detection signal is generated due to tracking movement of the object lens to the information track or to the tilting of the optical disk. This problem is explained below. FIGS. 1-3 show positions of the light beam on the photodetector in three cases and tracking error signals therefor. The abscissa of graphs of tracking error signal in the three cases illustrated at the right side in FIGS. 1-3 denotes a relative position X' of the center of the spot to the track. The tracking error signal shows a waveform schematically when the light spot crosses tracks. FIG. 1 shows a case where the object lens is located above the reference point (X=0). Because the light beam extends symmetrically relative to the division line between the two photosensitive areas, the tracking error signal changes symmetrically with no offset. On the other hand, in a second case where the object lens moves along X direction (or +X direction in the case shown in FIG. 2), the position of the light beam is shifted on the photodetector and the light beam distribution becomes asymmetrical. Then, the tracking error signal has a positive offset relative to the reference voltage. Tracking control performance is deteriorated if a value of (A-B)/(A+B) exceeds 20% where A and B denote the positive and negative maximum voltages of the tracking error signal.
Further, when an optical disk is tilted relative to the photodetector along .theta. direction, the light beam distribution becomes asymmetrical. FIG. 3 shows a case where an optical disk is tilted in -.theta. direction. The position of the light beam is also shifted on the photodetector in this case and the light beam distribution becomes asymmetrical relative to the division line. Then, the tracking error signal has an offset. Therefore, if the optical disk is tilted in +.theta. direction and the object lens is shifted in +X direction, the offset of the tracking error signal increases as a sum of the two causes. In an ordinary optical disk, tolerance of offtrack is about 0.1 .mu.m where off-track denotes a shift of position of zero tracking error signal relative to the track center. Tracking control for an optical disk is usually needed in a range of about 200 .mu.m of the shift of the object lens and in a range of about 1.degree. of tilt of the optical disk. However, in the prior art push-pull optical head, if X is 100 .mu.m and the tilt along .theta. direction is 0.5.degree., the value of (A-B)/(A+B) is 35% and off-track is 0.12 .mu.m. Therefore, the two values exceed the tolerances.
Because the prior art optical head using push-pull technique has the above-mentioned characteristics, an apparatus for reproducing an optical disk with the prior art optical head needs a means for carrying the optical head at a fast speed precisely for fast search to an object information track or for an optical disk having a large eccentricity such as about 100 .mu.m. Then, though the optical head of simple structure is installed, the optical disk reproducing apparatus becomes expensive. Further, because the means for carrying the optical head needs fast speed and high precision, it is not easy to increase tolerance for external shock and vibrations. Therefore, the optical head of push-pull technique is difficult to be installed in a portable optical disk reproducing apparatus.
A push-pull system is used in a focus and tracking error detector apparatus described in U.S. Pat. No. 5,113,386 by Whitehead et al. In order to make the tracking error output signal insensitive on side areas in a detector array, the detector array has a plurality of detectors, and masks or open areas are positioned at side areas outside the central areas including regions where the zeroth order diffraction beam overlaps the first order beams. However, this push-pull system does not solve the above-mentioned problem on the offset of the tracking error signal.
The present invention intends to solve the aforementioned problems, and its object is to provide an optical head which has a simple structure as the optical head of push-pull technique and reduces an offset of tracking error signal due to shift of the object lens and tilt of optical disk.